As originally designed and implemented, cable communication systems used only coaxial cable between a head-end controller (or distribution station) and the customer. Newer cable systems tend to be hybrids of optical fibers and coaxial cable; optical fiber being used for the long-haul portion of the network, and the coaxial cable being used to bring the communication from “curbside” into a home or office (in general, distances of less than a few thousand feet are spanned by a coaxial cable in such a network). With the growth of computer networks and a desire to provide real-time interactive services to the customer, the need has arisen for an efficient means to provide two-way (i.e., bidirectional) communication over an existing HFC network. A number of service providers currently furnish two-way services over cable. Providing customers with the ability to transmit data upstream requires service providers to reserve sections of the cable spectrum (bandwidth) or time slots for the upstream communication. The “upstream” voice signal originating with the customer is first packetized, usually forming a pulse code modulated (PCM) signal, at a predetermined bit rate. The bit rate is controlled by a known clock signal.
In prior art arrangements, the clock signal used for timing the collection of PCM samples of both inbound (downstream) and outbound (upstream) audio signals (e.g., voice signals) is derived from a “national” clock transmitted downstream by the cable modem termination system (CMTS). This signal is recovered by the local cable modem and divided down to the rate used within the modem (for example, 4.096 MHz). The use of a “national” signal thus provides system-wide synchronization. There is a clear advantage in using a single clock source for synchronizing the sampling at both ends of the telephone connection. For example, the common clock prevents clock slip or drift. Without a common clock, the sampling clocks at each customer's cable modem will drift apart and this drift will empty the “jitter buffer” at one end of a connection and overflow the jitter buffer on the other end of the connection. The one direction delay of the connection will change and audio samples may be lost. As a result, fax and modem calls may fail after a long period of time.
If the downstream signal is lost, the derived CMTS clock signal will have unpredictable characteristics. Failure of various components within the broadband terminal unit (BTI) at the customer's premise may result. In the past, a local reference clock has been used within the BTI to provide a backup clock upon failure of the CMTS-sent national clock. However, transition between the two clock sources will usually create glitches that could lead to improper operation of the DSP and/or the codec.
Thus, a need remains in the art for an arrangement for insuring a smooth transition from the national clock to the local clock upon loss of the national clock, as well as a smooth transition from the local clock to the national clock upon the recovery of the national clock.